Gallium nitride materials include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN). These materials are semiconductor compounds that have a relatively wide, direct bandgap which permits highly energetic electronic transitions to occur. Such electronic transitions can result in gallium nitride materials having a number of attractive properties including the ability to efficiently emit blue light, the ability to transmit signals at high frequency, and others. Accordingly, gallium nitride materials are being widely investigated in many microelectronic applications such as transistors, field emitters, and optoelectronic devices.
In many applications, gallium nitride materials are grown on a substrate. However, differences in the properties between gallium nitride materials and substrates can lead to difficulties in growing layers suitable for many applications. For example, gallium nitride (GaN) has a different thermal expansion coefficient (i.e., thermal expansion rate) than many substrate materials including sapphire, silicon carbide, and silicon. This difference in thermal expansion can lead to cracking of a gallium nitride layer deposited on such substrates when the structure is cooled, for example, during processing. The cracking phenomena can prevent gallium nitride materials from being suitable for use in many applications. Cracking can be particularly problematic for relatively thick (e.g., >0.5 micron) gallium nitride layers.
Gallium nitride (GaN) also has a different lattice constant than most substrate materials. The difference in lattice constant may lead to the formation of defects in gallium nitride material layers deposited on substrates. Such defects can impair the performance of devices formed using the gallium nitride material layers.
Prior art techniques have been developed to address crack formation and defect formation in gallium nitride materials deposited on sapphire substrates and silicon carbide substrates. Such techniques, for example, may involve depositing one or more buffer layers on the substrate and, then, depositing the gallium nitride material on the buffer layer(s).